1. Field of the Invention
The present invention relates to a centrifugal compressor, and more particularly, to a reverse rotation preventing structure of a centrifugal compressor, which is capable of preventing a reverse rotation in the compressor caused by a backward flow of a fluid due to a pressure difference between a suction side and a discharge side, when an operation of the compressor is suddenly stopped due to a power failure or a power shutdown caused by an abnormal operation of the compressor.
2. Description of the Related Art
Generally, a compressor is a machine that converts the mechanical energy into the compression energy of a compressive fluid, and is classified into a reciprocating type, a scroll type, a centrifugal (turbo) type and a vane (rotary) type.
Among them, the centrifugal compressor (so-called turbo compressor) sucks a fluid in an axial direction using a rotation force of an impeller and then discharges the fluid in a centrifugal direction, to thereby perform a compression operation. The centrifugal compressor is classified into two types, i.e., one-stage centrifugal compressor and two-stage centrifugal compressor, according to the number of the impellers and the compression chambers. Also, the centrifugal compressor is classified into two types, i.e., a back-to-back type and a face-to-face type, according to the type of the arrangement of the impellers.
Hereinafter, two-stage centrifugal compressor of the face-to-face type will be described with reference to FIG. 1.
Referring to FIG. 1, the centrifugal compressor of the face-to-face type includes a motor housing 1, a first bearing plate 2A and a second bearing plate 2B disposed at both ends of the motor housing 1, a shroud plate 3 mounted on an external face of the first bearing plate 2A, a first compression casing 4A mounted on an external face of the shroud plate 3, a bearing cover 5 mounted on an external face of the second bearing plate 2B, a volute casing 6 covering the bearing cover 5, a second compression casing 4B mounted on an external face of the volute casing 6, and a motor M mounted on the interior of the motor housing 1.
Here, a suction port SP is formed on one side of the motor housing 1 and a discharging port DP is formed on one side of the volute casing 6.
A first compression chamber SC1 is constituted with the shroud plate 3 and the first compression casing 4A, and a second compression chamber SC2 is constituted with the volute casing 6 and the second compression casing 4B.
The motor M providing the rotation force includes a stator MS, a rotor MR mounted on the interior of the stator MS, and a rotating shaft 7 rigidly fixed into the rotor MR. Also, both ends of the rotating shaft 7 of the motor M are passed through the first bearing plate 2A and the second bearing plate 2B, respectively. In particular, the rotating shaft 7 is supported in a radial direction by radial bearings 9A and 9B disposed inside the plates 2A and 2B and is supported in an axial direction by a thrust bearing 10.
A first impeller 8A and a second impeller 8B respectively disposed at the first compression chamber SC1 and the second compression chamber SC2 are attached to both ends of the rotating shaft 7 of the motor M. Further, the impellers 8A and 8B are arranged in the face-to-face type. In other words, the impellers 8A and 8B are arranged to face to each other in a direction of sucking the fluid. A reference number 13 that is not described represents a power supplying line.
According to the conventional two-stage centrifugal, or turbo, compressor of the face-to-face type, a low-temperature and low-pressure refrigerant is sucked into the suction port SP by the rotation of the rotating shaft 7 and the two impellers 8A and 8B. Here, the rotating shaft 7 is rotated by the driving of the motor M, and the two impellers 8A and 8B are connected to both ends of the rotary shaft 7. The sucked refrigerant flows into the first compression chamber SC1 through the first gas passage 11 and is primarily compressed by the first impeller 8A. The primarily compressed refrigerant, so-called provisionally compressed refrigerant, is sucked into the second compression chamber SC2 through the second gas passage 12 and is secondarily compressed by the second impeller 8B, thereby achieving an effective compression operation. The secondarily compressed refrigerant is gathered in the volute casing 6 and is discharged through the discharge port DP.
Therefore, in case where the centrifugal compressor is normally operated, the pressure difference between the suction port SP and the discharge port DP becomes high.
However, there is an occasion that an operation of the compressor is suddenly stopped due to a power failure during the normal compression operation or a power shutdown caused by an abnormal operation of the centrifugal compressor. At this time, to achieve the pressure equilibrium between the high-pressure discharge port DP and the low-pressure suction port SP, the refrigerant abruptly flows backward from the discharge port DP to the suction port SP through the refrigerant passage 12.
By the way, due to the abruptly backward flow of the refrigerant from the discharge port DP to the suction port SP, a reverse torque is instantaneously applied to the impellers 8A and 8B fixed on both ends of the rotating shaft 7. Further, the rotating shaft 7 is also rotated in a reverse direction.
At this time, in case where the impellers 8A and 8B and the rotating shaft 7 are rotated in the reverse direction, there is a problem that a performance of two dynamic-pressure air radial bearings 9A and 9B for supporting a radial load of the rotating shaft is degraded. Also, there is a problem that parts constituting the bearings 9A and 9B may be damaged.
Further, there is a disadvantage that a severe noise occurs due to the abruptly backward flow of the refrigerant from the high-pressure discharge port DP to the low-pressure suction port SP when the operation of the centrifugal compressor is suddenly stopped.
Furthermore, an impulse occurring during the backward flow of the refrigerant as well as the damages to the parts result in a shortening of a durability of the centrifugal compressor.